Drain Question

We are relocating the upstairs bathtub and lavatory. We are hoping to tie it into the drain for the kitchen sink in the basement below. The kitchen sink drain is 1.5" pipe.

Can we join into the 1.5" pipe? We can't go with 2", because we have 1.5" going through the concrete floor. Also, we assume that we need additional venting - the kitchen sink currently has a 1.5" vent.

Thank you very much for your help with this.

The kitchen sink would need 2.0"
The lav can use 1.5"
The tub needs 2.0"

The vertical usage for 1.5" would be okay for one lav.
Two lavs, or a tub and a lav should have 2.0"

Anytime you tie plumbing from a higher floor, the waste needs to tie in downstream of the lower fixture. That would mean tying them in lower then the tee for a sink. The sink needs to keep the vent. The upstairs fixtures will need venting, and those vents can tie into the lower vent at 42" above the floor on the second floor, (the highest point).

If the home is old, and used steel waste lines, near the floor I would expect to see a 2" cast iron hub. Pulling the 1.5" out of a cast iron hub would then allow the use of 2" pipe.